The short hand functions in the table’s right column simply are almost identical to those in the middle column. If one of the side arguments are specified with a character value, the relevant axis drawing function is used. You can however choose to use e.g. brackets_horisontal in place.
The axis drawing functions
The functions capped_horisontal or brackets_vertical returns a function that is called when ggplot2 prints the plot. In this package, we use ggplot2 to build the axes, then modify in place the return grobs.
The function is called by the coord objects when printing the plot, and is called with the arguments
scale_details, axis, scale, position, theme
The function should then return a grob. Some pointers to how it is used can be found in ggplot2’s help pages on ggproto (?"ggplot2-ggproto")
The arguments are as follows
scale_details: Details of the scales in ‘npc’ units (see grid::unit). In the example below, a secondary y-axis is used, so we find both y.major, etc., and y.sec.major, etc.
List of 20
$ x.range : num [1:2] 0.4 3.6
$ x.labels : chr [1:3] "a" "b" "c"
$ x.major : num [1:3] 0.187 0.5 0.812
$ x.minor : NULL
$ x.major_source : int [1:3] 1 2 3
$ x.minor_source : NULL
$ x.arrange : chr [1:2] "secondary" "primary"
$ y.range : num [1:2] -2.17 2.26
$ y.labels : chr [1:5] "-2" "-1" "0" "1" ...
$ y.major : num [1:5] 0.0378 0.2635 0.4892 0.7149 0.9406
$ y.minor : num [1:9] 0.0378 0.1506 0.2635 0.3763 0.4892 ...
$ y.major_source : num [1:5] -2 -1 0 1 2
$ y.minor_source : num [1:9] -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
$ y.sec.range : num [1:2] -10.8 11.3
$ y.sec.labels : chr [1:5] "-10" "-5" "0" "5" ...
$ y.sec.major : num [1:5] 0.038 0.263 0.489 0.715 0.941
$ y.sec.minor : num [1:9] 0.0378 0.1506 0.2635 0.3763 0.4892 ...
$ y.sec.major_source: num [1:5] -10 -5 0 5 10
$ y.sec.minor_source: num [1:9] -10 -7.5 -5 -2.5 0 2.5 5 7.5 10
$ y.arrange : chr [1:2] "primary" "secondary"
The secondary y-axis mulitplied the values by 5. Ticks are drawn at major coordinates, as are major grid lines. Minor grid lines, if the theme supports them, are drawn at minor. Observe the connection between y.label and y.major_source; it only becomes apparent when also observing the same for the x-axis. The x.major_source are the factor integers.
axis: Character, either "primary" or "secondary". Recall that the function is called per axis drawn.
scale: Character, either "x" or "y".
position: Character, one of the sides, "top", "right", "bottom", or "left".
theme: The plot’s theme. This is however not in absolute terms, e.g. so text sizes may be described in relative terms to the base size. To resolve a usable gp (graphic parameters, see ?grid::gpar) for a grob, use ggplot2:::element_render(theme, 'axis.text.x'), in which the second argument would resolve to the labels of the x-axis.
Brackets
The brackets comes in two orientations: brackets_horisontal and brackets_vertical. If you attempt to use a vertical bracket on a horisontal axis, it will fail with a undescriptive error.
The bracket functions accept a direction argument, which can be used to control which direction the end-points are pointing:
p <- ggplot(mpg, aes(cyl, hwy, colour=class)) +
geom_point(position=position_jitter(width=0.3)) +
scale_x_continuous(breaks=c(4,5,6,8), sec.axis=dup_axis()) +
scale_y_continuous(sec.axis=dup_axis()) +
coord_flex_cart(bottom=brackets_horisontal(),
top=brackets_horisontal(direction='down'),
left=brackets_vertical(),
right=brackets_vertical(direction='right')) +
my.theme
p
The look of the brackets are controlled via theme(axis.ticks). The length of the end-point are controlled via the theme theme(axis.ticks.length). If these needs to be specified for each margin, use the argument tick.length:
p <- ggplot(mpg, aes(cyl, hwy, colour=class)) +
geom_point(position=position_jitter(width=0.3)) +
coord_flex_cart(bottom=brackets_horisontal(tick.length=unit(0, 'cm')),
left=brackets_vertical(tick.length = unit(1, 'cm'))) +
my.theme
p
As shown above, using tick.length=unit(0, 'cm') results in a flat line.
